Opening rare earth ores with perfluoroalkanesulfonic acid

ABSTRACT

RARE EARTH METALS AND RELATED IONS SUCH AS THORIUM ARE EXTRACTED FROM ORES THEREOF BY TREATMENT WITH AQUEOUS SOLUTIONS OF PERFLUOROALKANESULFONIC ACIDS. TRIVALENT METALS ARE EXTRACTED PREFERENTIALLY, TETRAVALENT METALS TEND NOT TO DISSOLVE AS READILY.

United States Patent US. Cl. 252301.1 R 9 Claims ABSTRACT OF THE DISCLOSURE Rare earth metals and related ions such as thorium are extracted from ores thereof by treatment with aqueous solutions of perfluoroalkanesulfonic acids. Trivalent metals are extracted preferentially; tetravalent metals tend not to dissolve as readily.

This application is a continuation-impart of copending application Ser. No. 872,726 filed Oct. 30, 1969, now Pat. No. 3,615,169.

This invention relates to a process for a recovery of trivalent metals including rare earth metals and certain related or similar elements from minerals, ores, residues etc. by extraction with aqueous perfiuoroalkanesulfonic acids.

The rare earths usually occur in complex mixtures, for example, as the rather unusual minerals euxenite, xenotime, gadolinite, bastnasite and monazite. They are said to constitute a significant portion of the earths crust when considered together. Recovery of the metals (in ionic form) from these insoluble minerals is essential before they can be separated and used in various industrial processes. As a group, alloyed with iron or not, the metals are sometimes known by the German term mischmetall.

One process for isolation is that disclosed by Kremers et al. in US. Pat. 2,900,231 which utilizes inorganic, nonreducing acids such as dilute and concentrated sulfuric acid and nitric acids. It is, of course, very desirable to avoid acids whose salts with the rare earth metals are insoluble, e.g., HF.

It is an object of this invention to provide an effective and convenient method for the isolation of rare earth metals from their ores.

Other objects of the invention will be apparent from the disclosure hereinafter.

In accordance with the above and other objects of the invention it has been found that aqueous solutions of perfluoroalkanesulfonic acids are particularly adapted to the isolation of the rare earth metals. The salts are quite readily soluble in water and thus have an advantage over the sulfates which have a reverse solubility, that is, they are more soluble cold than hot and may precipitate when hot. Consequently solution in sulfuric acid requires a cold digestion which is not as efficient as a hot digestion. On the other hand, the process of the invention is carried out either hot or cold. Preferably it is carried out hot, for example, at the refluxing temperature of the solution.

The perliuoroalkanesulfonic acids are neither oxidizing nor reducing acids but are very strong acids. Prefluoromethanesulfonic acid is stronger than perchloric acid according to some measurements.

Broadly the perfluoroalkanesulfonic acids which are useful in the process of this invention are represented by the general formula:

RfSOZOH where R; is

'ice- The preferred acids may be represented by the formula R SO H where R, is a perfluoroalkyl group of from 1 to 18 carbon atoms. The most advantageous and preferred acids are, of course, those in which R; is 1 to 6 carbon atoms and particularly perfiuoromethanesulfonic acid. This is because the molecular weights are sufliciently low that the acids are readily soluble and furthermore because smaller amounts by weight will react with greater proportions of the rare earth metals. In particular trifiuoromethanesulfonic acid. CF SO H, is preferred. It is a considerably stronger acid than sulfuric acid and, as noted, is even stronger than perchloric acid.

The rare earth mineral, for example, bastnasite or monazite, which are the most plentiful ores, is digested with a solution of the selected perfluoroalkanesulfonic acid in water. The mineral need not be extremely finely ground and particles from 0.05 to 0.5 mm. are sufiiciently small although larger or smaller particles can be used. The concentration of the solution may be from 1% to 5% by weight and upwards to The more dilute solutions may act somewhat more slowly but are more economical of material. Solutions of 540% by weight are preferred. This reaction is suitably carried out in glass or glass lined equipment. Digestion may be carried out at any temperature desired but preferably the reaction is carried out while the solution is heated, for example, to approximately boiling point. Other acids may be present but appear to offer no particular advantage, e.g., nitric acid or sulfuric acid. Agitation is desirably continued inasmuch as this also causes a certain amount of grinding or further attrition of the particles themselves and is likely to enhance the yield and recovery. Time of digestion may be from relatively few minutes to a matter of many hours, for example, from about /2 to 20 hours. Shorter times 'may be used for solution of the more readily dissolved rare earth metals with a subsequent treatment for longer periods being used to recover greater amounts of the more difficulty dissolved metals. For example, an ore containing cerium when roasted will contain much of the cerium in the tetravalent form which is not easily dissolved. It may be advantageous to carry out a first extraction and then subsequently a mild reduction of the ore followed by further extraction for the recovery of the cerium in greater yield.

The recovery solution of perfluoroalkanesulfonate salts is conveniently processed to recover the rare earth metals for example, as their fluorides, by isolation of the perfiuoroalkanesulfonate salt and decomposition thereof as described in the parent of this application, Ser. No. 872,726, now Pat. No. 3,615,169. The perfiuoroalkanesulfonate salts of the rare earth metals are conveniently extracted from aqueous solution using non-hydrocarbon solvents, such as butyl alcohol, nitroethane, benzonitrile, methyl isobutyl ketone. Countercurrent flow extraction is, of course, the more efiicient process for extraction,

In somewhat more detail the steps are:

(1) The ore may or may not be calcined as desired and if desired it may be pulverized to the range of 0.05 to 0.5 mm. or smaller particle size.

(2) However it is prepared the ore is digested with an aqueous solution of perfiuoroalkanesulfonic acid preferably at a concentration of to 50% by weight.

(3) The slurry is filtered and the residue washed thoroughly. Until it is exhausted the residue can be reprocessed to obtain further amounts of products. The filtrate contains mostly trivalent rare earth values and some impurities such as iron, magnesium, strontium, calcium, barium and trace amounts of other metals which remain preponderantly in the residues. If yttrium and scandium are present they will normally be present because they behave as trivalent rare earths.

(4) The residues may be further processed directly and the extracts carried forward or they may be roasted and/or pulverized before being so treated.

(5) Aqueous solutions containing the rare earth values from all sources are pooled and may be treated in either of two ways:

(A) The solution may be evaporated to obtain a crude rare earth salt of the perfiuoroalkanesulfonic acid. The acid remaining is recovered by distillation under reduced pressure. The salt may be dissolved in a non-hydrocarbon organic solvent and further purified by extraction, etc.

(B) The pooled aqueous solution of salts is extracted with a non-hydrocarbon organic solvent to extract the rare earth perfiuoroalkanesulfonates either repeatedly or by countercurrent extraction. The organic solvent is then evaporated to yield a salt of higher purities than that obtained by evaporation in A above.

(6) Rare earth perfiuoroalkanesulfonate from either 5A or 5 B above may be pyrolyzed as described in the parent application to obtain pure rare earth fluorides.

The perfiuoroalkanesulfonates further may be converted to perfiuorocarboxylic acid or perfluoroacyl fluoride as described in my copending application Ser. No. 13,159 filed on even date herewith.

The process of the invention is more fully illustrated by the following examples.

EXAMPLE 1 Ten grams of calcined bastnasite ore, having a particle size of less than 0.1 mm., are added to 75 ml. of a heated solution of 33% CF SO H by weight in distilled water. The ore is digested at reflux temperature for 2.5 hours. The mixture is then diluted with 300 ml. of distilled water and warmed to about 50 C. The digested mixture is filtered by suction and the residue washed thoroughly with hot distilled water. The dried residue from the ore amounts to 5.3 gms. It still contains small amounts of rare earths as shown below in Table 1.

Evaporation of the filtrate to dryness provides a residue of rare earth (mischmetall) salts weighing 15.0 gms. A portion of these latter salts (0.8854' gm.) is placed in an alumina boat and pyrolyzed at 700 C. in an argon atmosphere. Rare earth fluorides (0.3542 gm.) are recovered.

The analyses of the original calcined ore, the unreacted residue and the final product for content of rare earth and alkaline earth cations as determined by emission spectroscopy (accurate to about i% of the value) are tabulated in Table 1. Here and subsequently these are estimated as percents from the spectrogram but are converted to and tabulated in terms of grams in a sample of 10 grams of original calcined ore. In some cases here and later the figures indicate more material in residue and soluble product than in original ore. This results from inaccuracies of the determination method.

TABLE 1 Grams Calcined Undissolved Soluble Recovery, ore residue product percent l Unexplained discrepancies probably associated with estimating such small amounts.

EXAMPLE 2 The procedure of Example 1 is followed using ten grams of raw bastnasite (not calcined). The analytical data are given in Table 2.

The procedure of Example 1 is followed using 20 grams of raw unpulverized monazite sand. Particles are mostly about 0.2 to 1 mm. The analytical data are given in Table 3.

TABLE 3 4.0

Grams Calclned Undissolved Soluble Recovery, ore residue product ercent EXAMPLE 4 Ten grams of pulverized calcined bastnasite are digested with 100 ml. of a by weight aqueous solution of CF SO H for five hours at reflux temperature. The digested slurry is diluted with 200 ml. water, filtered and the residue washed. The filtrate is extracted with 300 ml. of methylisobutyl ketone. The combined organic extracts are evaporated to dryness. The residue of rare earth perfluoromethanesulfonates weighs 10 grams, corresponding to 46% recovery. The water layer yields 12 gm. of residue part of which can be dissolved in fresh methyl isobutyl ketone.

In similar examples other solvents are employed with substantially the same results. The solvents include nitromethane, nitroethane, anisole, n-butanol.

Other experiments are carried out as described in Example 1. Various acids are used to digest the ore and the rare earth perfluoroalkanesulfonates are recovered in comparable yields. The acids used include C F SO H, (slFgO H, C F SO H, C F SO H and C F CF CF One hundred milligrams of calcined bastnasite as treated in Example 1 is added to a hot solution of Cl(CF SO H (1 ml.) in 2 ml. of water and digested for 15 minutes. The solution is filtered and the filtrate evaporated to give a residue of whitish salts, the rare earth or mischmetall ,8-

chloroperfluoroethane sulfonates. The salts are found to contain rare earth metals as obtained from other examples using bastnasite.

The last procedure is repeated using solutions of about 25-33% of perfluoropropane-2-sulfonic acid, ei-chloroperfluoropropane sulfonic acid, 'B-hydroperfluoroethane sulfonic acid, fi-hydroperfluoropentane sulfonic acid, perfluorocyclohexane sulfonic acid and perfluoromethylcyclohexane sulfonic acid. In each case when the digestedsolution is filtered and evaporated a residue of crystalline rare earth salt is obtained.

What is claimed is:

1. In a process for the extraction of rare earth values from an ore thereof the step comprising digesting said ore with an aqueous solution of perfluoroalkanesulfonic acid in a concentration range of from 1 to 100% by weight for times from about 30 minutes to 20 hours at a temperature from about 20 C. to the boiling point of the said solution whereby an aqueous extract of rare earth perfiuoroalkanesulfonates is obtained.

2. A process according to claim 1 wherein the perfluoroalkanesulfonic acid is trifiuoromethanesulfonic acid.

3. A process according to claim 1 wherein the ore is bastnasite.

4. A process according to claim 1 wherein the ore is monazite.

5. Aqueous extract of rare earth perfluoroalkanesulfonates obtained by the process of claim 1.

6. In a process according to claim 1 the additional step of reclaiming rare earth perfiuoroalkanesulfonates from aqueous extract by extraction thereof with a non-hydro- References Cited UNITED STATES PATENTS 2,874,176 2/1959 Mills et a1. 260-4291 2,988,421 6/1961 Kemp et al. 233l8 3,353,928 11/1967 Woyski et a1. 23-318 OTHER REFERENCES Hudlicky, Chemistry of Organic Fluorine Compounds, Macmillan Co., New York, 1962, pp. 153, 196, 309, 310. [QD/412/F1H8/196L] LELAND A. SEBASTIAN, Primary Examiner R. L. TATE, Assistant Examiner US. Cl. X.R.

UNETE STATES PMENT owrer QEBTWEQA'ED l QQREQTE Patent No. 5,725,296 Dated April "L 13975 Inventor(s) Karl F. Thom It is certified that error appears in the above-dde'nrified patent and that said Letters Patent are hereby corrected as shownbelow:

Column 2, line l -CF C-X should read T Y I Column 2, line 10 should read Column 2, line 62 "recovery" should read --recovered--..

Column 4, line 12 "En" should read --Ba- Column 4, line 9 "@0080" should reed O .0008

Signed and sealed this 25th day of December 1973.

[SEALl Attest:

EDWARD ML F LETCHERJR. RENE D. .TEGTMEYE R AttestingOfficer Acting Commissioner of Patents I FORM PO-IOSO (10-69) USCOMM-DC scam-pep 0.5. GOVERNMENY PRINTING OFFICEl l99 O-JSG-334 UNETED STATES MT N'E owmh QETWEQA'EFE F CQRRECWGN Patent No. 3,725,296 Dated Anril 5. 1973 Inventofls) Karl F. Thom It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shownrbelow:

Column 2, line l -CF -C-X should read T Y I Column 2, line 10 --(cs should read Column 2 line 62 "recovery" should read --recovered-- Column line 12 "En" should read --Ba-- Column P, line 9 "(1.0080 should reed --O .0008

Signed and sealed this 25th day'of December 1973.

(SEAL) Attest:

EDWARD M'Q'ELETCHER R. RENE D. .TEGTMEYER Attesting Officer Acting Commissioner of Patents Form po-aoso (10-69) USCOMM-DC 603764 69 a 11.5 covinuuzm animus omc: Ins o-Jss-JM 

